Post-flood nutrient management

As the floodwater begins to recede, corn and grain sorghum producers may wonder what management practices -- particularly in relation to nitrogen -- are appropriate. The magnitude of the loss of nitrogen will depend on soil type, field topography, fertilizer source, and time of nitrogen application, among others.

The management practices should also consider growth stage of the crop, whether the stand is sufficient to keep, and the crop yield potential if replanting is not needed. Farmers are encouraged to wait a few days before assessing the condition of the existing crop and decide on possible management practices.

The growing point in corn at the V4-V5 growth stage may still be below the soil level, so it is very sensitive to waterlogged conditions. Under normal growing conditions, the growing point is usually light yellow. A dark green growing point may be an indication of terminal damage. If the growing point is above ground, then the chances of survival increase significantly.

How much lost N?

The answer to this question will vary among, and within, individual fields and depends on many factors. The factor that may have the single greatest influence is when nitrogen fertilizer was applied.

The situations outlined below will not fit all fields, but should provide a general assessment that can be used to estimate nitrogen losses and develop a management plan.

Preplant nitrogen: Most of the corn in Arkansas was planted at least three or more weeks ago and received some amount of preplant nitrogen. Assuming that most preplant nitrogen is applied as urea, the majority of the nitrogen had been converted to nitrate by the time the heavy rains began.

Nitrate is susceptible to leaching on well-drained soils (sandy) or denitrification (loss as nitrogen gas) on poorly-drained soils. The amount of rainfall and how wet the soil is will dictate the fate of the nitrogen.

In fields that received high amounts of rainfall, but did not have standing water, a large proportion of the nitrate may have been leached or moved laterally with runoff. On fields that were saturated or had standing water, general estimates are that up to 5 percent of the soil nitrate will be lost for each day the soil is saturated or flooded.

Side-dressed nitrogen: Several growers had corn that was approaching the V6 or V7 stage and had applied their side-dress N to relatively dry soils in the days before the heavy rain started. This situation has some similarities to how preflood urea-N is managed for rice.

When urea was applied, 80 percent to 90 percent of the nitrogen is likely still in the field and plant available, regardless of how much rain occurred or whether the field was saturated.

For fields receiving UAN, nitrogen loss is expected to be greater. This is because about 25 percent of fertilizer nitrogen in UAN is in the nitrate form (60 percent to 65 percent of applied is available).

Suggested approach

Limited supply of nitrogen early in the season (before V6 stage) may not be as critical as later in the season. The crop does not need a large amount of nitrogen on or before the six-leaf stage.

By the time corn reaches the V6 stage, it has accumulated only 6 percent to 8 percent of its total N requirement. So, assuming that other factors have not hurt the yield potential, original plans should continue with side-dressing done so that applied nitrogen is available to plant roots by the time corn reaches six- to eight-leaf stage.

Nitrogen deficiencies during the V6 and V10 growth stages will affect corn yield potential. Under current conditions, and with the early-planted corn, it is possible that the plants have accumulated enough nitrogen to produce a plant of normal height.

However, it may not be enough to maximize the existing yield potential. If visual deficiency symptoms are observed after a few days of good weather, consider applying 100 pounds of urea as soon as possible.

Limited supply of nitrogen by the tassel stage typically results in small kernels. A yield increase from a pre-tassel (VT) nitrogen application may be more likely this season, especially in fields where substantial N loss has occurred and is not properly accounted for in the side-dress application or supplemental application.

This pre-tassel application would help ensure sufficient supply of nitrogen for grain filling. It may also help reduce yield variability associated with different amounts of N loss due to field topography.

What else can be done?

Would soil sampling help with estimating nitrogen losses? Nitrogen losses due to excess moisture tend to be variable, especially in soils of varying textures.

Arkansas does not have recommendations for the preside-dress soil nitrate test (PSNT), normally done in the first 35 days. Most states that do have PSNT recommendations suggest no additional N is needed when soil nitrate is greater than 20 to 25 ppm. Despite the absence of PSNT recommendations in Arkansas, information on soil nitrate in the top 12 inches of soil may be useful in assessing how much of the preplant or side-dress N remains in the field.

Tissue analysis may also help in identifying deficient corn plants. The nitrogen concentration of the uppermost developed corn leaves should be above 3 percent during the vegetative stage (V6 to Tassel).

Growers are encouraged to work with their county Extension agent in deciding how to best sample fields, interpret the information, and develop or revise their corn nitrogen management plans.

Are other nutrients affected by the wet conditions?

The main loss mechanism for phosphorus and potassium is erosion. Some P and K will be lost in runoff, but the amounts should be low if fertilizer was applied and incorporated several weeks before the recent rains. These losses are of greatest concern on well-drained (sandy) soils.

Sulfur (as sulfate) is very mobile in soils, especially in sandy soils. Fields should be monitored for visual symptoms of sulfur deficiency. Sulfur deficiencies symptoms tend to appear in young tissue as this nutrient is not translocated from older tissue parts, but may appear as a uniform yellowing of the entire corn seedling before V6. If a sulfur deficiency is confirmed, then 50 to 75 pounds per acre of ammonium sulfate should be applied.